When Margot Kidder died in May, she was remembered as the actress who brought the fictional reporter Lois Lane to life in a series of blockbuster Superman movies in the 1970s and ’80s.

Her obituary in The New York Times did not specify a cause of death. But on Wednesday the Park County coroner’s office in Montana, where she lived, revealed that the cause was suicide. A statement provided by the coroner to The Associated Press said that Ms. Kidder “died as a result of a self-inflicted drug and alcohol overdose.”

Ms. Kidder’s daughter, Maggie McGuane, said she knew that her mother had taken her own life.

“It’s a big relief that the truth is out there,” she told The A.P. “It’s important to be open and honest so there’s not a cloud of shame in dealing with this.”

In the month after Ms. Kidder died at age 69 on May 13 in Livingston, Mont., the designer Kate Spade and the chef Anthony Bourdain both took their own lives, prompting widespread conversations about how to prevent what has become a national public health crisis.

In a recent report, the Centers for Disease Control and Prevention found that the suicide rate in the United States was steadily rising. It is now the 10th leading cause of death, and one of only three leading causes that are on the rise, the report said.

Ms. Kidder was born in Canada and spent decades acting in movies and television shows before and after “Superman,” including “Some Kind of Hero,” “The L Word” and “Smallville.” She was also a political activist who was arrested in Washington in 2011 at a protest over the proposed Keystone XL pipeline.

And she was known for her work as a mental wellness advocate, in part because of her own struggles with mental health.

A serious car accident in 1990 left Ms. Kidder temporarily unable to work, putting her in debt. And after a public breakdown in 1996, she was found to have bipolar disorder.

She spoke openly about how she had wandered around Los Angeles for days during that episode, and she said in 2008 that she did not like the words “mental illness.”